1. Field of the Invention
The invention relates to a continuously operating press for the production of chipboards, fiberboards and plywood boards.
2. Description of the Related Art
In the case of presses operating in the high-pressure range, development is moving toward exerting higher surface pressures on the material being pressed. This is especially true if the purpose is to produce highly compressed chipboards, where the pressures range from 55 bar and above. With rolling support provided by means of rolling bars, these high pressures increasingly result in "Hertzian stresses" on the surfaces of the heated platens. On the other hand, the heated platens have to be produced from a weldable steel material, because it is necessary to weld in inserts appropriately at the deflection channels and generally seals at the outer end faces. Weldable steels have, however, only a limited carbon content, which normally results in a surface hardness of about 180 to 190 Brinell. Additional surface hardness treatments merely produce an increased Brinell hardness in the range from 200 to 220 Brinell. With the large dimensions of the heated platens (for example 2.5 m.times.10 to 15 m long), there is a thermal distortion which occurs during the course of the heat treatment for increasing hardness. Thus, there is a risk in such heat treatment processes that, if the heated platens are not rolled with a level surface area, the relatively thin hardened layer is removed in the subsequent grinding machining processes, thereby penetrating and exposing layers of lesser Brinell hardness.
In the practical operation of continuous presses using such heated platens, the above stated condition has the effect that considerable wear occurs on the heated platen's supporting surfaces after operating hours of about 3000 to 6000 hours. This wear does not result so much in a general removal of the supporting surface, but instead, grooves (with peak and valley) are formed transversely to the through-running direction of the material to be pressed. These grooves correspond approximately to the spacing of the rolling bars. This results in increased running noises, and with increasing wear, produces critical vibrations in the overall press system. At a surface pressure of 50 bar, the "Hertzian stress" with use of rolling bars (in the diameter range around 20 mm) lies at about 200 Brinell. Thus, with the slightest disturbances, for example, in a regular and uniform lubrication distribution system, the system operates unreliably as a result of which the wear phenomena described above occur.
It is of disadvantageous significance in this case that the orthogonal running of the rolling bars in the pressing area is not ensured and that it is possible for the rolling bars to run into each other and even be destroyed.